Auto Gas Permeability Testing in Fruit and Vegetable Preservation

This article briefs on the methods and mechanisms of preservation and storage for fruits and vegetables.

Abstract: This article briefs on the methods and mechanisms of preservation and storage for fruits and vegetables.
Keywords: preservation of fruit and vegetable, low temperature, permeability, data fitting
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People often focus their attentions on the guarantee period when buying food. Guarantee period is a term used for the safety of food. Food within this period can be eaten safely. However, with the improvement of living standard, people began to pay close attention to the freshness of food. Therefore, the concept of food preservation came into being. Food preservation, which has a higher requirement than quality guarantee, refers to that on the basis of safety insurance, the nutrition, color, texture and flavor can also be guaranteed.
1. Preservation of Fruit and Vegetable
Fruits and vegetables are the most seasonal crops with bigger demand at present. A lot of documents point out that the main reason for loss of fruits and vegetable is the improper storage after plucking. Now, seasonality of fruits and vegetables are becoming less obvious, which is closely related with the general application of preservative technology.
Fruits and vegetables can be stored in many ways. At present the commonly used methods for preservation or quality guarantee are chemical preservation, freeze preservation, sterilization by vacuum and high temperature to guarantee the quality, vacuum e and filling nitrogen, complex air adjusting and natural bio-preservation.
2. Preservation Theory
The course of storage is mainly influenced by two factors: aerobic fungus and oxidative reaction. Both of them need oxygen gas. Therefore, oxygen gas content of the environment should be reduced to prolong their shelf life or to maintain the quality. But O2 is essential in the process of package. This is because fruits and vegetables need O2 for breathing after plucking (such as consuming O2 and producing CO2), and the anaerobic respiration caused by lacking of O2 will accelerate their spoilage. To keep the freshness and commercial property of fruits and vegetables to the maximum extent, and to prolong their storage and shelf life, the measure of controlling storage conditions such as the concentration of humidity, oxygen gas, carbon dioxide and ethylene is recommended. In this way, we can inhibit their respiration and metabolism; reduce the decaying, plant diseases and insect pests and loss of moisture.
3. Auto Gas Permeability Testing of Preservative Film under Low Temperature
There are mainly three kinds of plastic wraps sold on the market at present: PVDC, PE and PVC. PE is mainly used for fruits and vegetables packaging and PVC and PVDC are mainly used for the package of cold cooked food. Relying on the permeable property of plastic wraps, food preservation is realized by utilize the breathing of fruits and vegetables to the maximum extent to reduce O2 and increase CO2 (oxygen density cannot be reduced to the extent of anaerobic respiration). In this way, respiration consumption is inhabited by postpone senility.
The application of plastic wrap is closely related with its permeability parameters. Choosing suitable plastic wrap combining with cold storage is the commonly used preservative method. Whether the permeability is suitable is of vital importance to the cost and quality of preservation. Some fruits and vegetables are cold resistant, some should be protected against cold. However, the one that can be stored under normal temperature are still rare. The commonly used storage temperature is between 0°C-5°C. Unless there is special specification, film permeability data are all tested under standard condition (23°C). Because fluctuation of temperature can influence permeability testing remarkably (detailed information can refer to the article named Influence of Temperature Fluctuation on Polymer Permeability updated on January 21st, 2005 in Labthink Forum), permeability of plastic wrap tested under normal temperature cannot represent its permeability in storage environment.
Although the lower limit of some permeability tester can reach about 0°C , the state of specimen and instrument will be influenced by low temperature to some extent and operators cannot perform the testing in low temperature environment. Therefore, how to obtain permeability data of plastic wrap in low temperature environment becomes a problem in the field of fruits and vegetable preservation pressing for solution.
Now this problem can be easily solved through the data fitting function of Labthink PARAM VAC-V1. Data fitting is not a kind of simple mathematic estimation, but a mathematic computational process in which permeability parameters under special temperature is calculated according to the known testing data under normal temperature through data fitting. The advantage of this method is that there is no need performing the test under low temperature and all the data needed for data fitting can be obtained under normal experimental condition. Data fitting is not selective to material. For example, under normal testing environment of 30 °C, 35 °C, 40 °C (these temperatures can be realized and maintained by the automatic temperature control function of VAC-V1), Labthink PARAM VAC-V1 is used to test oxygen permeability of PC film with a thickness of 125 μ m, the testing data is listed in table 1:
Table1. PC film testing data
Testing temperature /O2 transmission rate/O2 permeability coefficient
°C / cm3/m2•24h•0.1MPa / 10-11cm3•cm/cm2•s•cmHg
30/608.748 /11.59
Through data fitting of the above data, O2 permeability coefficient of 0 °C is 5.62×10-11 cm3•cm/cm2•s•cmHg, and the O2 transmission rate (OTR) is 295.441cm3/m2•24h•0.1Mpa. Permeability of 0°C is about two times that of 30°C. Construct the data of O2 transmission rate for PC film between -20°C and 50°C obtained through data fitting in Excel, we can get figure 1, in which the rate of increase for PC film permeability gradually slowed with the reducing of temperature.
But it is not always the case that permeability of 0°C is about two times that of 30°C to all kinds of film. For example, test the O2 transmission rate of film A (unknown material)under 23°C, 30°C, 35°C, 40°C, 45°C with PARAM VAC-V1(detailed data please refer to the article Influence of Temperature Fluctuation on Polymer Permeability). The O2 permeability coefficient of 0°C calculated through data fitting of the former results is 0.0697×10-11cm3•cm/cm2•s•cmHg and O2 transmission rate is 5.717cm3/m2•24h•0.1Mpa. But the O2 permeability coefficient for film A at 30°C is 0.3150×10-11cm3•cm/cm2•s•cmHg, and the O2 transmission rate is 25.862cm3/m2•24h•0.1Mpa. As the temperature reduces to 0°C, permeability of film A has improved more than four times comparing with that of 30°C.
With the reducing of temperature, film permeability usually shows a tendency of increase. But the rate of increase is not identical. Some increase quickly, while some are very steady. Therefore guidance only provided by the permeability data under normal temperature is very limited to the overall evaluation of fruits and vegetable preservation.
4. Summary
Plastic wrap is one of the commonly used methods in fruits and vegetable preservation. And it is always used combining with low temperature storage. Therefore, obtain the permeability data of plastic wrap under low temperature is very important to the research of preservative mechanism and effect.

Further information please feel free to contact Shirley Wu via, skype:shirley-labthink, or (tel)0086-531-85068709.

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